GUI event processing basics

　Event processing can be simply understood this way. When an event occurs, the program must respond according to this event. For example, we made a window that can change the background color through a button. When we click the button, an event is generated, and the program will respond according to this event, that is, to change the background color.

　So how does the program respond? This requires the event listener ActionListener, which is an interface that contains the actionPerformed method (that is, the operation performed based on the event), so we need to implement this interface (implement the actionPerformed method in the interface) to make a listener object. , and use the button to register the listener object, so that when the button is clicked, the listener will be called to perform the response.

                                     Running results

Code (starting from line 42 to implement the interface):

package buttonPanel;

import java.awt.*;
import java.awt.event.*; //事件监听器接口ActionListener的位置。
import javax.swing.*;

public class ButtonFrame extends JFrame {
    private ButtonPanel buttonPanel;
    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;
    
    public ButtonFrame() {
        setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
        setLocationByPlatform(true);
        
        //构造按钮
        JButton redButton = new JButton("RED");
        JButton yellowButton = new JButton("YELLOW");
        JButton blueButton = new JButton("BLUE");
        
        buttonPanel = new ButtonPanel();
        
        //添加按钮到面板
        buttonPanel.add(redButton);
        buttonPanel.add(yellowButton);
        buttonPanel.add(blueButton);
        
        add(buttonPanel);
        
        //构造对应颜色的动作监听器
        ColorAction redAction = new ColorAction(Color.red);
        ColorAction yellowAction = new ColorAction(Color.yellow);
        ColorAction blueAction = new ColorAction(Color.blue);
        
        //每个按钮注册对应的监听器
        redButton.addActionListener(redAction);                     
        yellowButton.addActionListener(yellowAction);
        blueButton.addActionListener(blueAction);
    }
    
    //为了方便调用buttonPanel，将ColorAction作为ButtonFrame的内部类。
    private class ColorAction implements ActionListener {
        private Color backgroundColor;
        public ColorAction(Color c) {
            backgroundColor = c;
        }
        public void actionPerformed(ActionEvent event) {
            buttonPanel.setBackground(backgroundColor);
        }
    }
    
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new ButtonFrame();
                frame.setTitle("ColorButton");
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}

class ButtonPanel extends JPanel {
    private static final int DEFAUT_WIDTH = 300;
    private static final int DEFAUT_HEIGHT = 200;

    @Override
    protected void paintComponent(Graphics g) {
        g.create();
        super.paintComponent(g);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(DEFAUT_WIDTH,DEFAUT_HEIGHT);
    }
}

In the above code, in order to facilitate the listener to call buttonPanel, ColorAction is used as the internal class of ButtonFrame. If you separate the ColorAction class, you need to pass the buttonPanel to the ColorAction. The implementation is as follows:

package buttonPanel2;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ButtonFrame2 extends JFrame {
    private ButtonPanel buttonPanel;
    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;
    
    public ButtonFrame2() {
        setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
        setLocationByPlatform(true);
        
        JButton redButton = new JButton("RED");
        JButton yellowButton = new JButton("YELLOW");
        JButton blueButton = new JButton("BLUE");
        
        buttonPanel = new ButtonPanel();
        
        buttonPanel.add(redButton);
        buttonPanel.add(yellowButton);
        buttonPanel.add(blueButton);
        
        add(buttonPanel);
        
        //将此对象通过this传到ColorAction的构造器。
        ColorAction redAction = new ColorAction(this,Color.red);
        ColorAction yellowAction = new ColorAction(this,Color.yellow);
        ColorAction blueAction = new ColorAction(this,Color.blue);
        
        redButton.addActionListener(redAction);
        yellowButton.addActionListener(yellowAction);
        blueButton.addActionListener(blueAction);
    }
    
    public void setButtonPanelsBackground(Color backgroundColor) {
        buttonPanel.setBackground(backgroundColor);
    }
    
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new ButtonFrame2();
                frame.setTitle("ColorButton");
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}

class ColorAction implements ActionListener {
    private ButtonFrame2 buttonFrame;
    private Color backgroundColor;
    
    //通过构造器的方法把ButtonFrame2对象传过来，这个对象包含了成员变量buttonPanel，以便对其更换背景色。
    public ColorAction(ButtonFrame2 buttonFrame,Color c) {
        this.buttonFrame = buttonFrame; //this.buttonFrame只是对象管理者，管理的还是ButtonFrame的对象frame。
        backgroundColor = c;
    }
    public void actionPerformed(ActionEvent event) {
        buttonFrame.setButtonPanelsBackground(backgroundColor);
        //这是我们在ButtonFrame2中添加的新方法。
    }
}

class ButtonPanel extends JPanel {
    private static final int DEFAUT_WIDTH = 300;
    private static final int DEFAUT_HEIGHT = 200;
    
    public ButtonPanel() {
        setBackground(Color.pink);
    }

    @Override
    protected void paintComponent(Graphics g) {
        g.create();
        super.paintComponent(g);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(DEFAUT_WIDTH,DEFAUT_HEIGHT);
    }
}

ButtonFrame2

There is a flaw in the code, that is, there is code duplication when constructing the button, adding the button to the panel, constructing the listener of the corresponding color, and registering the listener. In order to avoid code duplication, we can create a makeButton method to include these repeated operations, as follows:

package buttonPanel3;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ButtonFrame3 extends JFrame {
    private ButtonPanel buttonPanel;
    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;
    
    public ButtonFrame3() {
        setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
        setLocationByPlatform(true);
        
        buttonPanel = new ButtonPanel();        
        add(buttonPanel);
        
        makeButton("RED",Color.red);
        makeButton("YELLOW",Color.yellow);
        makeButton("BLUE",Color.blue);
    }
    
    //为了避免代码重复，我们将重复的操作放在这个函数里。
    public void makeButton(String name,final Color bg) {
        JButton button = new JButton(name);
        buttonPanel.add(button);
        button.addActionListener(new ActionListener() { //可以new一个接口出来，但是后面必须接花括号实现内部方法。
            public void actionPerformed(ActionEvent event) {
                buttonPanel.setBackground(bg);
            }
        });
    }
    
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new ButtonFrame3();
                frame.setTitle("ColorButton");
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}

class ButtonPanel extends JPanel {
    private static final int DEFAUT_WIDTH = 300;
    private static final int DEFAUT_HEIGHT = 200;

    @Override
    protected void paintComponent(Graphics g) {
        g.create();
        super.paintComponent(g);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(DEFAUT_WIDTH,DEFAUT_HEIGHT);
    }
}

ButtonFrame3

In the code, the listener is only called once, that is, when addActionListener(). So we don't need to make a separate class for the listener. Instead, we only need to create a new ActionListener interface directly when using the listener, and implement the interface method in curly braces.